1. Technical Field
The present invention relates generally to electric vehicles and, in particular, to an electric vehicle using a switched reluctance motor as a power plant.
2. Description of Related Art
With the increase in gasoline prices, and concerns over global warming, there is a significant interest in electric vehicles. While a number of different types of electric motors have been considered for use in electric vehicle applications, consideration has recently turned to the potential use of switched reluctance motors as the power plant for an electric vehicle.
Reluctance motors are well known in the art. These machines operate on the tendency of the machine's rotor to move to a position where the reluctance with respect to the stator is minimized (in other words, where the inductance is maximized). This position of minimized reluctance occurs where the rotor pole is aligned with an energized stator pole. When operated as a motor, energizing the stator pole generates a magnetic field attracting the rotor pole towards the stator pole. This magnetic attraction produces a torque causing the rotor to rotate and move towards the minimized reluctance position.
Both single-phase and multi-phase switched reluctance motors have been considered for electric vehicle applications. The present disclosure focuses on single-phase machines.
Reference is now made to FIGS. 1A and 1B which illustrate the general configuration and operation of a simple single phase switched reluctance motor of the 6/6 topology. The reference to “6/6” indicates that the machine has six rotor poles and six stator poles. The reference to “single-phase” indicates that there is only one stator energizing phase, and thus each of the six poles on the stator are energized simultaneously.
The stator 10 includes six poles 12. The rotor 18 is mounted to a shaft 20, and the shaft is supported by a housing and bearings (not shown) that allow for rotational movement of the rotor relative to the stator 10. The rotor 18 also includes six poles 22. The stator poles 12 and rotor poles 22 are salient poles, as is known in the art.
Each stator pole 12 is wound with a winding 14. The windings 14 for the six stator poles 12 are electrically connected in parallel and current is supplied thereto from a switched power supply 16. The winding direction for each stator pole winding 14 is indicated using an “×” and “●” nomenclature, where “×” indicates movement of charge into the page, and “●” indicates movement of charge out of the page. So, it will be noted with the windings 14 oriented as illustrated in FIGS. 1A and 1B, the magnetic field orientation of the stator poles when actuated alternates /N-S-N-S-N-S/ with respect to each stator pole around the circumference of the stator 10.
The magnetic flux paths 17 are shown with respect to the actuated stator poles 12. These paths flow from a first stator pole, cross the air gap to a first rotor pole, and flow from the first rotor pole through the web of the rotor to a second rotor pole adjacent the first rotor pole, cross the air gap to a second stator pole adjacent to the first stator pole, and flow from the second stator pole back to the first stator pole.
FIG. 1A shows the approximate angular orientation of the rotor 18 when a switched power supply 16 that is coupled to the windings 14 of the stator 12 poles may be actuated. Current is supplied to the stator pole windings 14 so as to simultaneously energize the six poles 12 of the stator 10. The six rotor poles 22 are attracted to the energized stator poles 12, producing a torque 24 on the shaft 20 and causing the rotor to rotate. The rotor poles 22 move towards the energized stator poles 12 in an effort to minimize the reluctance.
As the rotor poles 22 move towards the position of minimized reluctance (i.e., when the rotor pole 22 is aligned with the stator pole 12) as shown in FIG. 1B, the switched power supply 16 is de-actuated. Angular momentum is preserved and the rotor continues to rotate such that the rotor pole 22 passes by the de-energized stator pole 12. After a delay period which allows the rotor pole 22 to move sufficiently away from the stator pole 12 (i.e., move closer to the next stator pole), the switched power supply 16 is actuated again (see, FIG. 1A), and the process repeats.
It will be noted that proper operation of the motor is dependent on the timing of switched power supply 16 actuation and thus the actuation of the stator poles. That timing of actuation is driven by the angular position of the rotor poles relative to the stator poles. Thus, the motor further includes an angular position sensor 26 coupled to the shaft 20 to detect the angular position of the rotor poles relative to the stator poles. The angular position information output from the angular position sensor 26 is supplied to the switched power supply 16 to assist in controlling the timing of switched power supply 16 actuation of the stator poles 12.
Single-phase motors are believed to have limited use as an electric vehicle power plant because of concerns with, among other issues, start-up, limited maximum output torque, variations in output torque (known as torque ripple), energy and heat dissipation, and noise. However, single-phase motors advantageously need a relatively more simple control system than is used in multi-phase reluctance motors, and are preferred over multi-phase motors in many applications for this reason. There is accordingly a need in the art for an improved single-phase switched reluctance motor which addresses the limitations and concerns of prior art single-phase configurations while maintaining the advantages of simple control. There is further a need for a single-phase configuration which can support sufficient torque output in an electric vehicle application.